The present invention relates to clearance control in a turbine, such as a gas turbine.
Clearance in a turbine typically refers to the between the rotor of the turbine and the stator that surrounds the rotor. In a gas turbine, the rotor is typically an axial turbine having rows of buckets each mounted on a turbine wheel. The stator in a gas turbine is a casing that includes an inner annular shell supporting annular shrouds that surround the rows of buckets and rows nozzles between the bucket rows. Clearance is between the tips of the rotating buckets and annular shrouds.
Clearance is needed to allow the buckets to rotate without rubbing against the shrouds. If the clearance is too great, combustion gases leak over the tips of the buckets and do not drive the rotation of the turbine. If the clearance is too small, the tips of the buckets rub against the shroud and may cause vibration that damages the turbine.
Clearance is needed whenever the turbine buckets rotate, including while the turbine heats up during startup, while the gas turbine is hot during full speed, full load (FSFL) operation, and as the turbine cools as it shuts down. The turbine is typically formed of metal components having various heat expansion rates. In particular, the turbine wheels, buckets on the wheels and annular shells around the buckets expand and contract at different rates as the turbine heats up and cools down. Due to different rates of thermal expansion, clearance could increase or shrink as the gas turbine heats and cools.
Control systems and techniques are conventionally used on gas turbines to ensure that clearance never becomes too small during all stages of operation and does not become too large during extended periods of operation, especially at FSFL. Conventional clearance control systems and techniques may include cooling systems mounted on external skids adjacent the gas turbine, complex sensing and actuation systems for the cooling system, flow rerouting of compressed air form the compressor, and other assemblies. Conventional clearance control systems and techniques tend to be active in that they adjust the amount of a cooling fluid flowing through the shell or buckets.
Some conventional clearance systems are actuated in response to a certain operating conditions, such as at pinch points which occur when clearance is the smallest. For example, additional heating of the casing shell may be used to increase the clearance at a pinch point. Despite these conventional systems, there remains a long felt need for a clearance control system and scheme that is robust and economical.